Fine composite metal particles and their production method, micro-bodies, and magnetic beads

ABSTRACT

Fine composite metal particle comprising a metal core and a coating layer of carbon, and being obtained by reducing metal oxide powder with carbon powder.

This is a divisional of application Ser. No. 11/681,059 filed Mar. 1,2007, now U.S. Pat. No. 7,892,316, which is a divisional of applicationSer. No. 10/934,515 filed Sep. 7, 2004, now U.S. Pat. No. 7,285,329. Theentire disclosure of the prior applications, application Ser. Nos.11/681,059 and 10/934,515 is considered part of the disclosure of theaccompanying divisional application and is hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to fine composite metal particles andtheir production method, which may be used, for instance, for magneticrecording media such as magnetic tapes, magnetic recording disks, etc.,electronic devices such as noise-suppressing sheets, inductors, yokes,etc., magnetic beads for extracting and separating DNA, proteincomponents, cells, etc.

BACKGROUND OF THE INVENTION

As electronic devices has become smaller and lighter in weight withhigher performance, materials for the electronic devices are required tohave higher properties. One of methods for achieving such object is tomake the materials as small as a nano-size level. In magnetic productsformed by compacting magnetic powder, the use of fine magnetic powder isexpected to improve soft or hard magnetic properties.

In magnetic recording tapes having hard magnetic particles coated onsubstrates, for instance, both the reduction of a magnetic particle sizeand the improvement of magnetic properties are required to increasetheir recording density. The magnetic particles have conventionally beenferrite powder, which suffers from low signal intensity because of smallmagnetization. To obtain sufficient output, magnetic metal particles ofFe and/or Co are suitable. However, when the particle size of metalparticles is made smaller than 1 μm or less for higher recordingdensity, the oxidation reaction of metal particles vigorously occurs inthe air because they are vulnerable to oxidation, resulting in thedeterioration of magnetization.

To improve the oxidation resistance of fine metal particles containingFe and/or Co, proposals were made to coat the fine magnetic metalparticles with ferrite layers (for instance, JP 2000-30920 A), or tocoat Fe powder with graphite (for instance, JP 9-143502 A). However, themetal oxide coating disclosed in JP 2000-30920 A is disadvantageous inconsiderably oxidizing the metal particles. The coating of metalparticles with graphite as disclosed in JP 9-143502 A needs a heattreatment at as high a temperature as 1600° C. to 2800° C. to meltcarbon, disadvantageous for industrial use.

When magnetic metal powder is used in the form of moldings particularlyin high-frequency applications, electrical insulation should be securedbetween magnetic metal particles to improve properties. For thispurpose, each metal particle should be coated with a high-resistancematerial.

Proposed as a method for solving these problems is to coat metalparticles with high-resistance boron nitride (BN) (see InternationalJournal of Inorganic Materials 3 2001, p. 597, 2001). BN is a materialusable for crucibles, having a melting point of 3000° C. excellent inhigh-temperature stability, low reactivity to metals and goodinsulation. The coating of metal particles with BN can be carried out by(1) heating a mixed powder of a metal and B by arc discharge in anitrogen atmosphere, (2) heating a mixed powder of a metal and B in amixed atmosphere of hydrogen and ammonia, or (3) heat-treating a mixtureof a metal nitrate, urea and boric acid in a hydrogen atmosphere.

However, the above methods (1)-(3) suffer from the followingdisadvantages. Specifically, the above methods (1) and (2) suffer fromthe risk of burning due to rapid oxidation when handling ultrafine metalparticles of 1 μm or less. The method (1) suffers from low productivitybecause of arc discharge, and is disadvantageous for industrial usebecause of high reaction temperatures near 2000° C. The productionmethod (3) is likely to generate a harmful gas (NO_(x)) because of thethermal decomposition of metal nitrates. In addition, the hydrogen gasused in the methods (2) and (3) is easily exploded, unsuitable forindustrial use. These methods (2) and (3) suffer from extremely lowproductivity.

In addition, conventional coated metal particles have deterioratedsaturation magnetization because part of metal particles are modifiedwith coating materials. Thus, it is difficult to use fine particlesproduced by the conventional technologies for biochemical applicationssuch as the extraction of DNA and proteins, magnetic recording media,etc.

Recently, fine magnetic particles have become used in medical diagnosisand biological examination. For instance, the use of superparamagneticmetal oxide particles as carriers for binding nucleic acids is proposed(JP 2001-78761 A). The superparamagnetic metal oxide exhibitsmagnetization only when an external magnetic field is applied. Becausemagnetic particles are exposed to acidic or alkaline solutions in theabove applications, their surfaces should be chemically stable. Inaddition, antibodies for binding target substances should be easilyattached to their surfaces. When magnetic powder is used as carriers forextracting nucleic acids, the metal or metal oxide powder is coated withsilicon oxide (JP 2000-256388 A). According to this method, onlycoatings or fine particles of silicon oxide are used to cover the metalor metal oxide powder. Silicon oxide is formed by the hydrolysis ofsilicon alkoxides or by using condensed sodium silicate.

The magnetic particles for magnetic beads should have as small aparticle size as predominantly 1 to 10 nm to exhibit superparamagnetism.Accordingly, an extremely small force is induced in magnetic particlesby an external magnetic field, failing to gather the particlesefficiently. Because of a weak attraction force by a magnetic field,once gathered magnetic particles are likely to flow out together with adischarged solution. In the step of extracting nucleic acids withmagnetic powder having a silicon oxide coating or fine silicon oxideparticles on a magnetic metal core, the metal is likely to be dissolvedin a solvent or oxidized, resulting in the deterioration of magneticproperties. The metal dissolved in a solvent forms a complex with abuffer solution, hindering the extraction of DNA. When the core is madeof a metal oxide, the magnetic powder has extremely lower magneticproperties than when the core is made of a magnetic metal, resulting inlower efficiency in the extraction of nucleic acids.

OBJECT OF THE INVENTION

Accordingly, an object of the present invention is to provide finecomposite metal particles with high saturation magnetization andchemical stability and excellent in other properties, and a method forproducing such fine composite metal particles.

SUMMARY OF THE INVENTION

The fine composite metal particles according to one embodiment of thepresent invention have an average particle size of 1 μm or less, eachfine composite metal particle comprising a metal core and a carboncoating layer, and being obtained by reducing metal oxide powder withcarbon powder.

Each of the fine composite metal particles according to anotherembodiment of the present invention comprises a metal core, a carbonlayer covering the metal core, and wire-shaped or tubular carbonmicro-bodies, which are formed by reducing metal oxide powder in anon-oxidizing atmosphere.

The fine composite metal particles according to a further embodiment ofthe present invention have an average particle size of 1 μm or less,each fine composite metal particle comprising (a) an iron-based metalcore comprising 1% or more and less than 50% by mass of at least oneelement selected from the group consisting of Al, As, Be, Cr, Ga, Ge,Mo, P, Sb, Si, Sn, Ti, V, W and Zn, and having a main structural phaseof α-Fe, and (b) a coating layer mainly composed of carbon and/or boronnitride, the fine composite metal particles being obtained by reducingiron oxide powder.

Each of the nano-sized, spherical, composite metal particles accordingto a still further embodiment of the present invention comprises aniron-based metal core comprising Co and/or Ni, and a coating layerhaving a thickness of 1 to 40 nm, the mass ratios of Co/Fe and Ni/Febeing 0.3 to 0.82, and 0.01 to 0.5, respectively.

Each of the nano-sized, spherical, composite metal particles accordingto a still further embodiment of the present invention comprises aniron-based metal core comprising Co and/or Ni, and a coating layerhaving a thickness of 1 to 40 nm, a ratio of the intensity I (111) of a(111) peak of γ-Fe having a face-centered cubic crystal structure to theintensity I (110) of a (110) peak of α-Fe having a body-centered cubiccrystal structure is 0.2 or less in an X-ray diffraction pattern.

Each of the fine composite metal particles according to a still furtherembodiment of the present invention comprises a metal core comprising amagnetic metal as a main component and having an average particle sizeof 10 μm or less, and a multilayer coating of 2 or more differentinorganic materials.

In the above fine composite metal particles, carbon on a surface of eachmetal core is preferably composed mainly of graphite with 2 or morecrystal lattice planes. Carbon on a surface of each metal corepreferably has a thickness of 100 nm or less.

The metal core is preferably composed mainly of a magnetic metal; andthe saturation magnetization of the fine composite metal particles ispreferably 10% or more of that of the magnetic metal.

Increase in an oxygen content (% by mass) by a heat treatment at ahumidity of 100%, a temperature of 120° C. and 1 atm for 24 hours ispreferably 50% or less relative to before the heat treatment.

A ratio of the intensity I (111) of a (111) peak of γ-Fe having aface-centered cubic crystal structure to the intensity I (110) of a(110) peak of α-Fe having a body-centered cubic crystal structure ispreferably 0.3 or less in an X-ray diffraction pattern.

An innermost inorganic layer partially or entirely covering the metalcore is preferably mainly formed by carbon and/or boron nitride. Theinorganic material preferably has 2 or more crystal lattice planes. Theinnermost inorganic layer preferably has a thickness of 100 nm or less.

The innermost inorganic layer preferably comprises at least one elementselected from the group consisting of Si, V, Ti, Al, Nb, Zr and Cr.

The inorganic layer outside the innermost inorganic layer is preferablysubstantially composed of silicon oxide or gold. The silicon oxide layerpreferably has a thickness of 400 nm or less. The outside inorganiclayer is preferably coated with a resin. The resin is preferably coatedwith a silicon oxide layer. The resin layer is preferably sandwiched bythe innermost inorganic layer and the outside inorganic layer. Theoutside inorganic layer preferably has at least one functional groupselected from the group consisting of —NH₂, —OH and —COOH on itssurface.

The method for producing fine composite metal particles coated withcarbon according to a still further embodiment of the present inventioncomprises heat-treating a mixture of metal oxide powder and carbonpowder in a non-oxidizing atmosphere.

The method for producing fine composite metal particles each comprisinga metal core, a carbon layer covering the metal core, and wire-shaped ortubular carbon micro-bodies according to a still further embodiment ofthe present invention comprises heat-treating a mixture of metal oxidepowder and carbon powder in a non-oxidizing atmosphere to reduce themetal oxide.

The method for producing fine composite metal particles according to astill further embodiment of the present invention comprises the steps ofmixing oxide powder of a magnetic metal with at least one ofboron-containing powder and carbon powder to provide a mixed powder,heat-treating the mixed powder in a non-oxidizing atmosphere to producefine metal particles each coated with a layer based on carbon and/orboron nitride, and further coating the resultant coated fine metalparticles with an inorganic material.

The method for producing fine composite metal particles according to astill further embodiment of the present invention comprises the steps ofmixing oxide powder of a magnetic metal with powder containing at leastone element selected from the group consisting of Si, V, Ti, Al, Nb, Zrand Cr to provide a mixed powder, heat-treating the mixed powder in anon-oxidizing atmosphere to produce fine metal particles coated with alayer based on at least one of the elements, and further coating theresultant coated fine metal particles with an inorganic material.

The method for producing fine composite metal particles according to astill further embodiment of the present invention comprises the steps ofmixing oxide powder of a magnetic metal, at least one ofboron-containing powder and carbon powder, and powder containing atleast one element selected from the group consisting of Al, As, Be, Cr,Ga, Ge, Mo, P, Sb, Si, Sn, Ti, V, W and Zn to provide a mixed powder;heat-treating the mixed powder in a non-oxidizing atmosphere to producefine metal particles containing at least one of the above elements andcoated with a layer comprising carbon and/or boron nitride, and furthercoating the resultant coated fine metal particles with an inorganicmaterial.

In the production method of fine composite metal particles, the heattreatment is conducted at a temperature of 600 to 1600° C. The layer ofthe inorganic material is preferably substantially composed of siliconoxide formed by hydrolyzing silicon alkoxide. The silicon oxide layer ispreferably formed from silicon alkoxide, water, a catalyst and anelectrolyte in an alcohol solvent. After forming the inorganic layer,the fine composite metal particles are preferably further coated with anamino-group-containing silane coupling agent to introduce an —NH₂ group.

The wire-shaped or tubular micro-bodies of graphite-phase carbonaccording to a still further embodiment of the present invention areformed by reducing metal oxide powder in a non-oxidizing atmosphere.

The magnetic beads for extracting biosubstances according to a stillfurther embodiment of the present invention comprise the above finecomposite metal particles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the X-ray diffraction pattern of the mixedpowder before the heat treatment;

FIG. 2 is a graph showing the X-ray diffraction pattern of the mixedpowder after the heat treatment;

FIG. 3 is an electron photomicrograph showing the particle of thepresent invention structure;

FIG. 4 is a schematic view for explaining the structure of the particleshown in FIG. 3;

FIG. 5 is an electron photomicrograph showing the particle of thepresent invention structure;

FIG. 6 is a schematic view for explaining the structure of the particleshown in FIG. 5;

FIG. 7 is an electron photomicrograph showing the particle of thepresent invention structure;

FIG. 8 is a schematic view for explaining the structure of the particleshown in FIG. 7;

FIG. 9 is a graph showing the X-ray diffraction pattern of the mixedpowder before the heat treatment;

FIG. 10 is a graph showing the X-ray diffraction pattern of the mixedpowder after the heat treatment;

FIG. 11 is an electron photomicrograph showing the particle of thepresent invention structure;

FIG. 12 is a schematic view for explaining the structure of the particleshown in FIG. 11;

FIG. 13 is an electron photomicrograph showing the particle of thepresent invention structure;

FIG. 14 is a schematic view for explaining the structure of the particleshown in FIG. 13;

FIG. 15 is a graph showing the X-ray diffraction patterns of the powdersamples;

FIG. 16 is a graph showing the X-ray diffraction patterns of the Fe—Coparticles;

FIG. 17 is a graph showing the relation between the thickness of asilicon oxide layer and electric resistance in the fine composite metalparticles;

FIG. 18 is a TEM photograph showing the multilayer-coated fine compositemetal particle;

FIG. 19 is a schematic view for explaining the structure of themultilayer-coated fine composite metal particle shown in FIG. 18;

FIG. 20 is a TEM photograph showing part of the multilayer-coated finecomposite metal particle of FIG. 18 in an enlarged manner;

FIG. 21 is a schematic view for explaining the structure of themultilayer-coated fine composite metal particle shown in FIG. 20;

FIG. 22 is a graph showing the relation between the concentration ofimmunoglobulin in a protein suspension and the amount of immunoglobulinattached to the fine composite metal particles;

FIG. 23 is a TEM photograph showing the multilayer-coated fine compositemetal particle;

FIG. 24 is a schematic view for explaining the structure of the finecomposite metal particle shown in FIG. 23;

FIG. 25 is a photograph of the fine composite metal particles of thepresent invention in a fluorescent-labeling method, which was taken by afluorescent inverted microscope;

FIG. 26 is a schematic view corresponding to the photograph of FIG. 25;

FIG. 27 is a photograph showing the results of an electrophoresisexperiment with a DNA-extracting liquid, using the fine composite metalparticles of the present invention; and

FIG. 28 is a schematic view corresponding to the photograph of FIG. 27.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Each fine composite metal particle of the present invention comprises ametal core and a coating layer, having an average particle size of 10 μmor less. The coating layer comprises 2 or more different inorganicmaterials. The above particle size range is necessary for good softmagnetic properties. When the average particle size exceeds 10 μm, thefine composite metal particles have low dispersibility in a solvent,resulting in precipitation in a short period of time. The averageparticle size of the fine composite metal particles is desirably in arange of 0.1 to 5 μm to have extremely high dispersibility in a solvent.Though not particularly restrictive, the lower limit of the averageparticle size is preferably 10 nm, so that Fe, Co and Ni particles mayhave critical sizes for superparamagnetism.

[1] Metal Core

The metal core is preferably composed of at least one magnetictransition metal selected from the group consisting of Fe, Co and Ni. Itmay be formed by Fe, Co or Ni alone or their alloys such as Fe—Co alloysor Fe—Ni alloys, or alloys of Fe, Co and/or Ni containing othertransition metals such as Cr, Ti, Nb, Si, Zr, etc.

Particularly in the case of iron-based alloys, elements X added to Feare preferably those providing a stable a phase even at as hightemperatures as 1000° C. or higher when alloyed with Fe. The iron-basedalloy preferably comprises at least one element X selected from thegroup consisting of Al, As, Be, Cr, Ga, Ge, Mo, P, Sb, Si, Sn, Ti, V, Wand Zn. Because finer metal Fe particles precipitate a paramagnetic γphase having a face-centered cubic crystal structure, resulting indecreased magnetic properties, the element X is preferably 1 to 50% bymass in the fine composite metal particles having an average particlesize of 1 μm or less to suppress the γ phase. The fine composite metalparticles containing such element may be obtained by heat-treating amixture of at least one of boron-containing powder and carbon powder,oxide powder of a magnetic metal, and powder containing at least oneelement selected from the group consisting of Al, As, Be, Cr, Ga, Ge,Mo, P, Sb, Si, Sn, Ti, V, W and Zn in a non-oxidizing atmosphere. Theaddition of the element X forms fine α-Fe phase particles, resulting inan intensity ratio I (111)/I (110) of 0.3 or less in an X-raydiffraction pattern, wherein I (111) is the intensity of a (111)diffraction peak of γ-Fe having a face-centered cubic crystal structure,and I (110) is the intensity of a (110) diffraction peak of α-Fe havinga body-centered cubic crystal structure. Thus, even the fine compositemetal particles having an average particle size of 1 μm or less, further100 nm or less, have high saturation magnetization. In the finecomposite metal particle comprising a metal core having a multilayercoating of inorganic materials according to the present invention, asmall metal core contributes to high saturation magnetization.

The fine metal particles may have Fe as a main component, and contain Coand/or Ni. The average particle size of the fine composite metalparticles is particularly preferably in a range of 1 to 10000 nm. Inthis case, the thickness of the coating layer is preferably in a rangeof 1 to 40 nm to have high saturation magnetization. The amounts of Coand Ni are preferably such that mass ratios of Co/Fe and Ni/Fe are0.3-0.82 and 0.01-0.5, respectively, in the nano-sized, iron-basedparticles. In this binary composition, the alloying of Fe with Coelevates the transition temperature of an α phase to a γ phase, a highertemperature phase, thereby stabilizing the α phase and suppressing theprecipitation of the γ-Fe phase. When the mass ratio of Co to Fe is lessthan 0.3, no addition effect of Co is expected. On the other hand, whenCo/Fe exceeds 0.82, the saturation magnetization becomes less than 120Am²/kg. The addition of Co in the above preferred range suppresses theprecipitation of the γ phase, resulting in a ratio I (111)/I (110) of0.2 or less and thus high saturation magnetization, wherein I (111) isthe intensity of a (111) diffraction peak of a face-centered cubiccrystal structure (corresponding to the γ phase), and I (110) is theintensity of a (110) diffraction peak of a body-centered cubic crystalstructure (corresponding to the a phase) in an X-ray diffractionpattern.

The ternary Fe—Co—Ni composition has, in addition to the above effects,excellent corrosion resistance and soft magnetic properties, highsaturation magnetization, and low magnetostriction. When the ratio ofNiFe is less than 0.01 by mass, the fine metal particles have largemagnetostriction. On the other hand, when the ratio of NiFe is more than0.5 by mass, the fine metal particles have saturation magnetization ofless than 100 Am²/kg.

Because the Fe-based core containing Co and/or Ni has high saturationmagnetization, the metal core preferably has a multilayer coating ofinorganic materials, so that the whole coated particles have highsaturation magnetization. The metal core with suppressed γ-Fe, on whicha multilayer coating of 2 or more different inorganic materials isformed, is preferably as small as 1 μm or less in an average particlesize.

[2] Innermost Inorganic Coating

In the multilayer coating of 2 or more inorganic materials formed on themetal core, an inorganic layer (innermost layer) in direct contact withthe metal core for partial or entire covering is preferablysubstantially composed of carbon and/or boron nitride. Because carbonand boron nitride have excellent lubrication, the coating of the metalcore with carbon and/or boron nitride improves the dispersibility ofmetal particles. It should be noted that carbon as a simple substance iscategorized in the inorganic materials. Carbon has a 6-membered ringstructure peculiar to graphite, which is laminated in a laminar manner.Boron nitride also has a ring structure, which is laminated in a laminarmanner. These inorganic materials preferably cover the entire surface ofthe metal core uniformly, though they may partially cover the metal coreas long as oxidation can be prevented when exposed to the air.

The ring plane of the inorganic material in the inorganic layer coversthe metal core in a laminar manner. This is easily achieved by carbonhaving a graphite structure. In the case of boron nitride, it has ahexagonal crystal structure to achieve the above object. The inorganiclayer with such a laminar structure preferably has excellent chemicalstability, because of few branches such as dangling bonds, etc.

The innermost layer of carbon and/or boron nitride in contact with themetal core preferably has a thickness of 100 nm or less. When it has athickness exceeding 100 nm, the saturation magnetization decreasesbecause of an increased non-magnetic phase. The thickness of theinnermost layer is more preferably 50 nm or less, most preferably 40 nmor less.

The innermost inorganic layer preferably has 2 or more crystal latticeplanes laminated in a laminar manner to have excellent corrosionresistance. In the case of a single crystal lattice plane, the existenceof defects directly leads to decrease in corrosion resistance. Thethickness of the innermost layer is preferably 1 nm or more.

In addition to carbon and boron nitride, the innermost inorganic layermay be formed by an oxide or a nitride of at least one element (elementM) selected from the group consisting of Al, B, Ce, Co, Cr, Ga, Hf, In,Mn, Nb, Ti, V, Zr, Sc, Si, Y and Ta. Typical elements needing smallactivation energy for oxidation are Si, V, Ti, Al, Nb, Zr and Cr. Thus,the innermost layer may be formed by carbides, nitrides or oxides ofthese elements. Because the element M, which is more easily oxidizedthan Fe, Co and Ni, has a Gibbs free energy of forming an oxide, whichmeets the relation (1) described below, it can reduce oxides containingFe, Co and Ni.ΔG_((Fe,Co,Ni)-O)≧ΔG_(M-O)  (1),wherein ΔG_((Fe,Co,Ni)-O) is a Gibbs free energy of forming oxides ofFe, Co and Ni, and AG_(M-O) is a Gibbs free energy of forming an oxideof the element M.

When the iron oxide is Fe₂O₃, the oxides, whose ΔG_(M-O) is smaller thanΔG_(Fe2O3) (−740 kJ/mol), are Al₂O₃, As₂O₅, B₂O₃, CeO₂, Ce₂O₃, Ce₃O₄,Cr₂O₃, Ga₂O₃, HfO₂, In₂O₃, Mn₂O₃, Mn₃O₄, Nb₂O₅, TiO₂, Ti₂O₃, Ti₃O₅,V₂O₃, V₂O₅, V₃O₅, ZrO₂, Sc₂O₃, Y₂O₃, Ta₂O₅, rare earth oxides, etc.

[3] Formation of Metal Core and Innermost Inorganic Layer

The metal core and the innermost coating layer are produced byheat-treating fine oxide particles of Fe, Co, Ni, etc. together withcarbon powder or boron powder in a non-oxidizing atmosphere such as anitrogen gas, or a mixed gas of a nitrogen gas and an inert gas such asargon, etc.

Detailed explanation will be made, for instance, on fine composite metalparticles having an average particle size of 1 μm or less, each of whichcomprises a metal core coated with carbon. The fine composite metalparticles can be produced by heat-treating a mixture of oxide powder ofFe, Co, Ni, etc. and carbon powder in a non-oxidizing atmosphere. Bythis production method, the metal oxide is reduced, and graphite-basedcarbon with 2 or more crystal lattice planes is formed on the metalparticles. 2 or more laminar crystal lattice planes are desirable toprevent the metal particles from being oxidized. More desirably, thecarbon layer has 4 or more crystal lattice planes. The crystal latticeplanes of carbon are preferably formed along a surface of each metalparticle.

It appears that the transition metals, particularly Fe, Co and Ni, actas catalysts for forming the graphite layer. Accordingly, the abovemethod has an extremely simplified step, as compared with conventionalmethods conducting the formation of fine metal particles and the coatingof the fine metal particles with carbon by different steps, therebypreventing oxidation during the step. This method is effective to formfine composite metal particles, which are extremely active and thuseasily oxidized.

The average particle size of the fine composite metal particles isdesirably 0.001 to 10 μm, more desirably 0.001 to 1 μm, most desirably0.01 to 0.1 μm. Particularly when the average particle size is 0.1 μm orless, the carbon coating exhibits a remarkable effect of preventingoxidation, and such effect is secured in a range of 0.01 to 0.1 μm.Because the fine magnetic metal particles suffer from littledeterioration of magnetic properties by oxidation, they exhibitsufficient magnetization even when having multilayer coatings ofinorganic materials, optimum for magnetic beads, etc.

The carbon layer of the fine composite metal particles has a thicknessof preferably 100 nm or less, more preferably 50 nm or less, mostpreferably 40 nm or less. The magnetic metal core can be coated with athin carbon layer, the saturation magnetization of the fine compositemetal particles can be 10% or more and less than 100% of that of themagnetic metal. The fine composite metal particles coated with carbonhave such high corrosion resistance that increase in the oxygen contentafter a heat treatment at a humidity of 100%, a temperature of 120° C.and 1 atm for 24 hours is 50% by mass or less relative to before theheat treatment. Thus, chemically stable fine composite metal particlescan be obtained.

Carbon sources are suitably artificial or natural graphite, carbonblack, etc., though they may be carbon-containing compounds such ascoal, activated carbon, cokes, polymers such as aliphatic acids,polyvinyl alcohol, etc., B-C compounds, metal-containing carbides.Accordingly, it should be noted that the term “carbon powder” includesnot only powder of pure carbon, but also carbon-containing compoundpowder. The powder of pure carbon is most preferable to have ahigh-purity carbon coating layer.

The metal oxide powder preferably has an average particle size of 0.001to 10 μm, preferably 0.001 to 1 μm, particularly 0.01 to 0.1 μm. It isdifficult to produce metal oxide powder having an average particle sizeof less than 0.001 μm, and thus it is not practical. When the averageparticle size exceeds 10 μm, it is difficult to sufficiently reduce themetal oxide powder to its center, failing to obtain uniform metalparticles. The average particle size of carbon powder is preferably 0.01to 100 μm, more preferably 0.1 to 50 μm. Carbon powder of less than 0.1μm is too expensive. When the average particle size of carbon powderexceeds 100 μm, a uniform dispersion of the carbon powder in the mixedpowder cannot be obtained, failing to uniformly coat the metal particleswith carbon.

A mixing ratio of the metal oxide powder to the carbon powder ispreferably such that the carbon powder is 25 to 95% by mass. When thecarbon powder is less than 25% by mass, a sufficient reduction reactiondoes not occur because of insufficient carbon. On the other hand, whenthe amount of carbon powder exceeds 95% by mass, the volume ratio of themetal oxide powder to be reduced is impractically small.

The mixing of the metal oxide powder and the carbon powder may becarried out by a V-type mixer, a pulverizer such as a ball mill, arotational mixer, a mortar, etc. The mixed powder is charged into aheat-resistant crucible of alumina, boron nitride, graphite, etc.together with a transition metal oxide and heat-treated. The heattreatment atmosphere is a non-oxidizing atmosphere such as an inert gas,which may be a nitrogen gas, or a mixture of a nitrogen gas and anotherinert gas such as argon, etc. The heat treatment temperature ispreferably 600° C. to 1600° C., more preferably 900° C. to 1400° C. Thereduction reaction does not proceed at a temperature of lower than 600°C., and it takes too long time at a temperature of lower than 900° C.When it exceeds 1400° C. in an atmosphere free from oxygen, oxideceramics of the crucible are likely to be decomposed to dischargeoxygen, and an alumina crucible, for instance, is likely to be damaged.When it exceeds 1600° C., the crucible and adjacent facilities should bemade of heat-resistant materials, resulting in extremely high productioncost.

When the transition metal oxide and the carbon powder are heated at 600to 1600° C. in a non-oxidizing atmosphere to produce the fine compositemetal particles coated with carbon, the metal oxide acts as a catalystfor forming wire-shaped or tubular carbon micro-bodies having an averagediameter of 0.5 μm or less as a byproduct. The term “carbonmicro-bodies” includes nanotubes, nano-wires, nano-particles, and theiraggregates. The wire-shaped or tubular carbon micro-bodies may have agraphite phase. The tubular carbon micro-bodies may have nodes orbridges. The metal oxide is desirably a magnetic metal oxide, and inthis case, the resultant fine composite metal particles can magneticallybe separated from the carbon micro-bodies.

The carbon micro-bodies may be hollow or solid, having an averagediameter of desirably 0.01 to 0.5 μm, more desirably 0.05 to 0.5 μm,particularly 0.1 to 0.3 μm. The average diameter is determined from theouter diameters of the wire-shaped or tubular carbon micro-bodies. Whenthe fine particles have noncircular cross sections, their maximum outerdiameters and their minimum outer diameters are averaged to obtain theaverage particle size. When the carbon micro-body has a graduallychanging diameter in a longitudinal direction, the maximum and minimumouter diameters in a longitudinal direction are taken to determine amiddle diameter as its outer diameter. Part of the carbon micro-bodieslargely deviating from a wire or tubular shape should be ignored in thedetermination of their outer diameters. The average diameter of thewire-shaped or tubular carbon micro-bodies is determined by measuringthe outer diameters of the carbon micro-bodies in the number of N(N≧50)in an electron photomicrograph, and dividing the sum of the outerdiameters by N.

Detailed explanation will then be made on fine composite metal particleshaving an innermost layer (first layer) of at least one element Zselected from the group consisting of Si, V, Ti, Al, Nb, Zr and Cr. Amixture of the magnetic metal oxide powder and the element Z powder isheat-treated in a non-oxidizing atmosphere such as Ar, He, H₂, N₂, CO₂,NH₃ or their combination.

The fine composite metal particles comprising Fe as a main component andfurther containing Co and/or Ni may be produced by using a mixture ofiron oxide and oxide powder of Co and/or Ni, or by using a compositeoxide powder of Fe and Co and/or a composite oxide powder of Fe and Ni.The iron oxide may be Fe₂O₃, Fe₃O₄, FeO, etc., the Co oxide may beCO₂O₃, CO₃O₄, etc., and the Ni oxide may be NiO, etc. The compositeoxide of Fe and Co may be CoFe₂O₄, etc., and the composite oxide of Feand Ni may be NiFe₂O₄, etc.

The powder containing the element M includes powder of a simple elementM, and powder of carbides (M-C), borides (M-B) or nitrides (M-N). Theaverage particle size of the element M-containing powder is preferably 1to 10000 nm, more preferably 1 to 1000 nm, most preferably 10 to 100 nmfor a more efficient reduction reaction. Though B and As are called“metalloid,” they are included in the metal element here.

A mixing ratio of the oxide powder containing Fe and Co and/or Ni to thepowder containing the element M (at least one selected from the groupconsisting of Al, B, Ce, Co, Cr, Ga, Hf, In, Mn, Nb, Ti, V, Zr, Sc, Si,Y and Ta) is preferably close to a stoichiometric ratio to reduce theoxides of Fe and Co and/or Ni. More preferably, the powder containingthe element M is more than a stoichiometric amount. When the powdercontaining the element M is insufficient, the oxides of Fe and Co and/orNi are not sufficiently reduced by the heat treatment, resulting insintering of the element M particles and thus a bulky composite metalbody.

The heat treatment may be conducted in a stationary electric furnacehaving a tubular chamber; an electric furnace having a furnace tubemovable during the heat treatment, such as a rotary kiln, etc.; anapparatus for heating powder in a fluidized state; an apparatus forheating gravitationally falling fine particles by high-frequency plasma,etc. In any apparatus, the oxide powder is reduced to form the metalcore and the first layer simultaneously. An additional inorganic layeris formed on the first coating layer to provide the multilayer-coated,fine, composite metal particles.

The simultaneous formation of the metal core and the first layersuppresses the oxidation of the metal core. The first coating layermakes it possible to obtain fine composite metal particles havingextremely high corrosion resistance and oxidation resistance even frommetals having poor corrosion resistance and oxidation resistance. Whilea silicon oxide-based layer is formed on the first coating layer of eachfine composite metal particle, the metal core can be efficiencyprevented from deterioration. The fine composite metal particles coatedwith the silicon oxide-based layer on the first coating layer haveextremely high magnetic properties, corrosion resistance and oxidationresistance, when used as extraction media for nucleic acids.

[4] Inorganic Layer Outside Innermost Layer

The outermost layer of each fine composite metal particle is preferablya silicon oxide-based layer, not only to secure electric insulationbetween the particles, but also to have properties as an extractioncarrier of nucleic acids. Though other insulating inorganic materialsmay be used, silicon oxide is most practical from the aspect of massproduction at a low cost. Though the magnetic metal core is preferablycoated with an inert inorganic material resistant to oxidation, etc.,the outermost surfaces of the fine composite metal particles should beactive to biosubstances such as DNA, etc., when used as biosubstanceextraction media. From this aspect, the outermost layer is preferablycomposed of silicon oxide.

The silicon oxide may be obtained, for instance, by the hydrolysisreaction of silicon alkoxides. Specific examples of the siliconalkoxides are tetramethoxysilane, tetraethoxysilane,tetraisopropoxysilane, tetrabutoxysilane, methyltrimethoxysilane,methyltriethoxysilane, aminophenyltrimethoxysilane,aminopropyltrimethoxysilane,N-2-aminoethyl-3-aminopropyltrimethoxysilane,3-triethoxysilyl-N-(1,3-dimethylbutylidene) propylamine,N-2-aminoethyl-3-aminopropyltriethoxysilane,N-2-aminoethyl-3-aminopropylmethyldimethoxysilane,N-phenyl-3-aminopropyltriethoxysilane,N-phenyl-3-aminopropyltrimethoxysilane, aminopropyltriethoxysilane,dimethyldiethoxysilane, dimethyldimethoxysilane, tetrapropoxysilane,phenyltriethoxysilane, etc. Tetraethoxysilane is a preferable siliconalkoxide to provide silicon oxide (silica) by a controlled hydrolysisreaction with good reproducibility. In addition, tetraethoxysilaneprovides a highly insulating coating at a relatively low cost. Thesilicon alkoxide may be used alone or in combination. For instance,aminopropyltrimethoxysilane and tetraethoxysilane may be used to form asilicon oxide layer with improved corrosion resistance.

In addition to silicon oxide, the outermost layer may be made of suchelectrically insulating inorganic oxides as alumina, titania, zirconia,etc. These coating layers may be formed by the hydrolysis method ofmetal alkoxides.

In the case of using tetraethoxysilane, the fine composite metalparticles having the metal core coated with the first inorganic layerare dispersed in a solution of an alcohol, for instance, a lower alcoholsuch as ethanol, methanol, isopropanol, etc. To accelerate thehydrolysis reaction of tetraethoxysilane, an ammonia water is added as acatalyst. The ammonia water contains water in an amount equal to or morethan a theoretical amount for 100% hydrolyzing tetraethoxysilane.Specifically, water is 2 mol or more per 1 mol of tetraethoxysilane. Theamount of an alcohol solution used is preferably 100 to 10000 parts bymass per 100 parts by mass of tetraethoxysilane.

The amount of tetraethoxysilane used is preferably 5 to 80 parts bymass, more preferably 10 to 60 parts by mass, per 100 parts by mass ofthe fine composite metal particles. When tetraethoxysilane is less than5 parts by mass, it is difficult to uniformly coat a surface of eachfine composite metal particle with a silicon oxide layer. On the otherhand, when tetraethoxysilane is more than 80 parts by mass, fineparticles composed only of silicon oxide are formed in addition to thesilicon oxide layer covering the fine composite metal particles.

The amount of water used for the hydrolysis of tetraethoxysilane ispreferably 1 to 1000 parts by mass per 100 parts by mass oftetraethoxysilane. When it is less than 1 part by mass, the hydrolysisof tetraethoxysilane proceeds slowly, resulting in poor productionefficiency. On the other hand, when it exceeds 1000 parts by mass,separate spherical particles of silicon oxide are undesirably formed.

The amount of the ammonia water used as a catalyst is preferably 10 to100 parts by mass per 100 parts by mass of tetraethoxysilane, forinstance, when the concentration of the ammonia water is 28%. When it isless than 10 parts by mass, the ammonia water fails to exhibit acatalytic function. On the other hand, when it is more than 100 parts bymass, separate spherical particles of silicon oxide are undesirablyformed. The above dispersion of the fine composite metal particles isslightly alkaline with pH of about 11, because ammonia water and siliconalkoxide are contained. Accordingly, the metal particles may becorroded. However, the inorganic layer of each fine composite metalparticle formed simultaneously with the metal core prevents the metalcore from being corroded.

To coat the fine composite metal particles uniformly with silicon oxide,the alkoxide solution and the fine composite metal particles are fullymixed using a motor stirrer, a V-type mixer, a ball mill, a dissolver,or an ultrasonic machine, etc. The mixing should be conducted longerthan necessary for the hydrolysis of tetraethoxysilane reaction. Toincrease the intensity of the coating layer, it is preferable toheat-treat the coated fine composite metal particles.

The silicon oxide layer has an amorphous structure. The averagethickness of the silicon oxide layer is preferably determined such thatthe average thickness of the multilayer inorganic coating comprising theinnermost layer and the silicon oxide layer is 500 nm or less. To obtaina sufficient magnetic force, the saturation magnetization of the finecomposite metal particles is preferably 10% or more of that of themagnetic metal. When the average thickness of the multilayer inorganiccoating exceeds 500 nm, the fine composite metal particles have adecreased saturation magnetization. More preferably, the averagethickness of the silicon oxide layer is 100 nm or less. When the finecomposite metal particles of the present invention are used asbiosubstance-extracting media, which are called “magnetic beads,” theminimum thickness of the silicon oxide layer is 5 nm, such thatsufficient chemical properties are exhibited, for instance, its surfacepotential (ζ potential) defined from the aspect of an electric doublelayer is the same as that of silicon oxide.

The thickness of the coating layer is a distance between the surface ofthe metal core and the coating surface. The thickness of the coatinglayer is measured, for instance, by TEM. The TEM observation of a sampleparticle reveals that there is contrast between the metal core, theinnermost layer of carbon and/or boron nitride and the silicon oxidelayer, indicating that the carbon and/or boron nitride layer and thesilicon oxide layer are formed on the metal core surface. The averagethickness is determined from 10 or more of the fine composite metalparticles. With respect to each particle, the thickness of the layer ismeasured at 4 or more points, and averaged to determine the averagethickness.

When a periodically arranged lattice is not observed in the siliconoxide layer by high-resolution electron microscopic observation, it isconfirmed that the silicon oxide has an amorphous structure. Theformation of the silicon oxide layer on the fine metal particle surfacecan be confirmed, for instance, by element analysis such asenergy-dispersive X-ray fluorescence (EDX) analysis, etc., or infraredspectroscopy. The TEM observation of the fine composite metal particleand the EDX analysis of the coating layer can confirm that the layer iscomposed of silicon oxide. The infrared absorption spectrum of the finecomposite metal particles reveals that there is an absorption peak ofsilicon oxide at a wave number of 1250 to 1020 cm⁻¹, confirming theformation of the silicon oxide layer.

The average particle size of ultrafine particles can be determined, forinstance, by dispersing an ultrafine particle sample in a solvent,irradiating laser beams onto the sample to measure a particle sizedistribution by diffraction. A measured median diameter d50 is used asan average particle size. When the particle size is as small as 100 nmor less, a sample is observed by TEM or a scanning electron microscope(SEM) to take its electron photomicrograph, in which the sizes ofparticles are measured in an arbitrary area to obtain an averageparticle size. The average particle size is determined from 50 or moreof the measured particles. When the fine particles have noncircularcross sections, their maximum outer diameters and their minimum outerdiameters are averaged to obtain the average particle size.

When the silicon oxide layer is formed by the hydrolysis oftetraethoxysilane, the thickness of the silicon oxide layer depends onthe amounts of tetraethoxysilane, water and a catalyst. The excessamount of silicon oxide forms separate particles. The thickness of thesilicon oxide can be increased by adding an electrolyte. Specificexamples of the electrolyte are KCl, NaCl, LiCl, NaOH, etc. With acontrolled amount of the electrolyte, the thickness of the silicon oxidecan be adjusted in a range of 5 to 400 nm. The silicon oxide layerthicker than 400 nm decreases the saturation magnetization of the finecomposite metal particles. The more preferred thickness of the siliconoxide layer is 5 to 100 nm.

An inorganic material outside the innermost layer may be a gold layer inplace of the silicon oxide layer. The gold layer may further be formedon the silicon oxide layer. Fine colloidal gold particles need only beapplied to the coating layer, and the resultant gold layer may beheat-treated. The first coating layer may be plated with Ni, etc. andthen with Au.

[5] Resin Layer

The multilayer inorganic coating may be covered with a resin. The resincoating may be formed by the precipitation polymerization of monomerssuch as monofunctional vinyl monomers. The monofunctional vinyl monomersmay contain polyfunctional vinyl monomers in a range causingsubstantially no cross-linking, for instance, less than 0.5 mol % of thetotal monomers. The resin coating is particularly a polystyrene resincoating. The resin-coated fine composite metal particles may beaggregated to have a controlled secondary particle size. Theabove-described silicon oxide layer may be formed on the resin coating.

An intermediate layer of a resin may be formed between the innermostlayer in contact with the metal core and the outer inorganic layer.Specifically, the metal core may be coated with an innermost layer ofcarbon and/or boron nitride, or at least one element selected from thegroup consisting of Si, V, Ti, Al, Nb, Zr and Cr, a resin layer, andthen a silicon oxide layer. With the resin coating, the particle sizeand specific gravity of the fine composite metal particles can becontrolled.

The outermost layer is preferably a silicon oxide layer, a gold layer ora resin layer, though all the fine composite metal particles need not becoated with such layers. In addition, each fine particle is preferablycompletely coated, though all the fine composite metal particles neednot be coated. The coating ratio of the particles is preferably 90% ormore. The coating ratio is calculated by (n/N)×100(%), wherein N is thetotal number of particles, and n is the number of particles, 50% or moreof whose surfaces are coated with silicon oxide. When there are a smallpercentage of particles coated with silicon oxide, the effect ofimproving oxidation resistance and corrosion resistance is low. Thus,the coating ratio by silicon oxide is more preferably 95% or more. Thefine composite metal particles of the present invention having amultilayer coating of 2 or more different inorganic materials have suchgood corrosion resistance that the degradation of saturationmagnetization is 10% or less in a corrosion resistance test under theconditions of a temperature of 120° C. and a humidity of 100% for 12hours, exhibiting stable properties as magnetic beads.

[6] Surface Modification

The silicon oxide layer may be treated with amino-group-containingsilane coupling agents to have functional groups such as —NH₂, etc. Thesilane coupling agents may be specifically γ-aminopropyltrialkoxysilane,N-β-(aminoethyl)-γ-aminopropyltrialkoxysilane,N-β-(aminoethyl)-γ-aminopropyl methyl dialkoxysilane,N-phenyl-γ-aminopropyltrialkoxysilane, γ-aminopropyltriethoxysilane, orN-(β-aminoethyl)-γ-aminopropyltrimethoxysilane. With functional groupssuch as —NH₂, —OH, —COOH, etc. on the surface of the silicon oxidelayer, the fine composite metal particles have an improved function ofextracting biosubstances.

The fine composite metal particles of the present invention,particularly those having a silicon oxide layer and optionally asurface-modifying layer of —NH₂, —OH, —COOH, etc. outside the inorganiclayer, are chemically stable with high saturation magnetization.Accordingly, they are suitable for the extraction of biosubstances asso-called magnetic beads.

The present invention will be described in further detail referring toEXAMPLES below without intention of restricting the present inventionthereto.

Example 1

5 g of α-Fe₂O₃ powder (metal oxide powder) having an average particlesize of 0.03 μm and 5 g of carbon powder having an average particle sizeof 20 μm were mixed for 10 minutes in a V-type mixer. The resultantmixed powder was charged in a proper amount into a boron nitridecrucible, and the crucible was placed in a tubular furnace, in which themixed powder was heated from room temperature at a rate of 3° C./minkept at 1000° C. for 2 hours, and then cooled to room temperature at arate of 3° C./min. in a nitrogen gas stream at a flow rate of 2 L/minWith respect to each carbon powder before and after the heat treatment,an X-ray diffraction measurement (Cu, Kα lines) was conducted to obtaindiffraction patterns shown in FIG. 1 (before the heat treatment) andFIG. 2 (after the heat treatment). (002) Peaks of graphite and (110)peaks of α-Fe were mainly detected in the heat-treated powder. Themagnetic properties of the heat-treated powder were measured by asample-vibrating magnetometer (VSM). The results are shown in Table 1.The saturation magnetization was about 100 times larger in Example 1than in Comparative Example 1, and this result in combination with theresult of the X-ray diffraction measurement indicates that Fe₂O₃ wasreduced to Fe. After the heat-treated powder was placed at 100% RH, 120°C., and 1 atm for 24 hours in a pressure cooker test machine, the oxygencontent in the powder was analyzed by an analyzer of oxygen, nitrogenand hydrogen in metals (EMGA-1300 available from Horiba Ltd.). Theanalysis results of the oxygen content are shown in Table 2. There wasno increase in the oxygen content after the pressure cooker test,indicating that Fe powder was coated with a graphite layer.Incidentally, the above analysis of oxygen, nitrogen and hydrogen inmetals comprises charging 0.5 g of a powder sample into a graphitecrucible, rapidly heating the crucible to 2000 to 3000° C. to thermallydecompose the sample to generate a gas containing CO₂, N₂ and H₂O, anddetecting them by a chromatograph and a heat conductivity detector toanalyze the amounts of O, N and H.

Comparative Example 1

The mixed powder was heat-treated and subjected to an X-ray diffractionmeasurement, a VSM measurement and a pressure cooker test in the samemanner as in Example 1 except for changing the heat treatmenttemperature to 500° C. There was no change in an X-ray diffractionpattern before and after the heat treatment, suggesting that no reactionoccurred. The saturation magnetization was not substantially changedeither.

Comparative Example 2

Fe powder having an average particle size of 0.02 μm (ultrafineparticles available from Vacuum Metallurgical Co., Ltd.) was subjectedto a corrosion resistance test under the same conditions as inExample 1. The saturation magnetization was reduced to 75.4×10⁻⁶Wb·m/kg, about 30% of the pure iron level (about 260×10⁻⁶ Wb·m/kg).

TABLE 1 Saturation Magnetization Coercivity No. (Wb · m/kg) (kA/m)Example 1  131 × 10⁻⁶ 8.04 Comparative Example 1 1.30 × 10⁻⁶ 140Comparative Example 2 75.4 × 10⁻⁶ 135

TABLE 2 Amount of Oxygen (% by mass) Before Corrosion After CorrosionNo. Resistance Test Resistance Test Example 1 0.11 0.11 ComparativeExample 1 15.0 15.2 Comparative Example 2 10.8 27.5

FIG. 3 is an electron photomicrograph showing the structure of ultrafinemetal particle of Example 1, which was observed by TEM. In the TEMobservation, no work likely to damage or change the particles wascarried out at all except for necessary adjustment of the sample. FIG. 4is a slightly enlarged schematic view for explaining the structure inFIG. 3. Ultrafine metal particles 11 were α-Fe particles 12 eachentirely coated with a thin graphite layer 20. This coating layer 20effectively prevents the oxidation of each α-Fe particle 12. It ispresumed that projections 21, 21 b on the thin graphite layer 20 arethose grown from the graphite layer or foreign matter attached to thegraphite layer. Bright and dark stripes 13 a, 13 b, 13 c on the α-Feparticles 11 are an interference pattern (shown by phantom lines) due tothe substantially spherical shape of the α-Fe particles 11. Becauseboundaries 14 of the thin graphite layer are slightly unclear on bothends of the α-Fe particles, they are shown by dotted lines. This isbecause perfect focusing is difficult on substantially spherical shapes,not suggesting that the α-Fe particles 11 actually have obscure shapes.A region encircled by a dashed line is a collodion layer 15 for fixingthe ultrafine metal particles 11 to a sample holder for TEM observation.FIG. 4 indicates that three lines 16 are as long as 20 nm. A scale, onwhich the photograph of FIG. 3 was taken, is copied to FIG. 4.

FIG. 5 is a TEM photomicrograph showing the structure of the particle ofthe present invention, which shows a lower left portion in FIG. 3 in anenlarged manner to scrutinize the structure of the thin graphite layer20. FIG. 6 is a schematic view for explaining the structure of theparticle shown in FIG. 5. A thin graphite layer 20 is mainly constitutedby graphite crystals. For instance, lattice fringes 20 a, 20 b, 20 cwith substantially equal intervals indicate laminar lattice planes ofgraphite, substantially in parallel to the surface of the α-Fe particle12.

Because the α-Fe particle is spherical and has slight raggedness on itssurface, some part of the thin graphite layer is inclined to the surfaceof the α-Fe particle 12, like lattice fringes 20 f. The lattice fringes20 f generate other lattice fringes 20 e and further lattice fringes 20d. Though not shown in the figure, the lattice fringes 20 d aresubstantially in parallel to the surface of the α-Fe particle 12. Asshown in FIGS. 5 and 6, the laminar lattice planes have variedorientations, and some of them (satin portions) are obscure in theirorientations in the thin graphite layer. A substantially uniform thingraphite layer was formed, such that crystals grow with adjacentcrystals aligned and with surfaces in parallel to the surface of theα-Fe particle 12.

FIG. 6 schematically indicates part of lattice fringes by solid linesand dotted line in the thin graphite layer 20. Though not shown in thefigure, there are crystals even in blank portions. It is presumed thatthere is an intermediate layer 25 between the surface of the α-Feparticle 12 and the thin graphite layer 20, and its details areexplained referring to FIGS. 7 and 8.

FIG. 7 is a TEM photomicrograph showing the structure of the particle ofthe present invention, which shows the thin graphite layer 20 in FIG. 5in an enlarged manner. FIG. 8 is a slightly enlarged schematic viewshowing the structure in FIG. 7. The α-Fe particle 12 has latticefringes 24 along the arrow a, indicating crystal planes peculiar toα-Fe. The arrangement of Fe atoms in row is shown by dotted lines.Though the lattice fringes along the arrow a are disturbed near thesurface of the α-Fe particle 12, their ends constitute flat surfaces assites, from which graphite crystals gradually grow. This region iscalled an intermediate layer 25. The thin graphite layer 20 grows viathe intermediate layer 25.

If the α-Fe particle 12 has a relatively smooth spherical surface (flatsurface) as shown in FIG. 8, the laminar lattice planes 22 of graphiteregularly grow in parallel along the arrow d, resulting in a dense andextremely thin coating. The term “dense” used herein means that thecoating has portions in which regular lattice planes are laminated. 15to 17 laminar lattice planes were laminated in a direction perpendicularto the arrow d to constitute an extremely thin coating. It seems thatthe intermediate layer 25 was as thick as 1 to 2 layers in the laminarlattice planes 22 of graphite. When graphite crystal planes juststarting to grow are included in the intermediate layer, theintermediate layer 25 is as thick as about 2 to 4 layers in the laminarlattice planes 22 of graphite.

It is presumed that dislocations 23 are portions having discontinuity inthe crystal structure because of uneven crystal growth. When growth in aportion delays by one layer due to lattice defects and then resumes inthe course of growth of laminar lattices, it is likely that a latticeplane of the (n−1)-th layer in the portion merges with a lattice planeof the n-th layer in an adjacent portion into one plane (dislocatedplane). When the formation of dislocated planes is scarce, the resultantlayer is a uniform, dense, thin layer as a whole. Though there areprojections 21 b on the thin graphite layer 20, their clear photographwas not taken. Accordingly, they are shown by dotted lines in FIG. 7.

Example 2

5 g of α-Fe₂O₃ powder (metal oxide powder) having an average particlesize of 0.03 μm and 5 g of carbon powder having an average particle sizeof 20 μm were mixed for 10 minutes in a V-type mixer. A proper amount ofthe resultant mixed powder charged into a boron nitride crucible washeated from room temperature at a rate of 3° C./min. in a nitrogen gasstream at a flow rate of 2 L/min. in a tubular furnace, kept at 1000° C.for 2 hours, and cooled to room temperature at a rate of 3° C./min. inthe furnace.

With respect to each carbon powder before and after the heat treatment,an X-ray diffraction measurement (Cu, Kα lines) was conducted to obtaindiffraction patterns shown in FIG. 9 (before the heat treatment) andFIG. 10 (after the heat treatment). (002) Peaks of graphite and (110)peaks of α-Fe were mainly detected in the heat-treated powder. In FIG.10, white circles denote peaks corresponding to the crystal structure ofgraphite, black squares denote peaks corresponding to the crystalstructure of α-Fe, the axis of abscissas indicates a diffraction angle2θ(°), and the axis of ordinates indicates an X-ray diffractionintensity (cps). A maximum-intensity peak of graphite has a half-widthof about 0.2°, indicating that the heat-treated powder has goodcrystallinity. With respect to the powder shown in FIGS. 9 and 10(before and after the heat treatment), the measurement of magneticproperties by VSM revealed that the saturation magnetization was1.30×10⁻⁶ Wb·m/kg before the heat treatment, and 131×10⁻⁶ Wb·m/kg afterthe heat treatment, indicating that Fe₂O₃ was reduced to Fe.

The TEM observation of the heat-treated powder confirmed tubular carbonmicro-bodies as shown in FIG. 11. FIG. 11 is an electron photomicrographshowing the structure of the particle of the present invention. Thecomposition analysis by EDX revealed that black particles at ends of thecarbon micro-bodies are Fe, and that the tubes are C (carbon).

FIG. 12 is a schematic view corresponding to the particle structure inthe photograph of FIG. 11. A carbon micro-body 201 is a carbon tube 203grown from an Fe particle 202 in a curved manner. There are pluralitiesof nodes 204 in the tube 203, regions surrounded by opposing nodes andtube wall 203 b constituting cavities 205. A tip end 206 of the tube 203is closed. The curved carbon micro-body 201 appears to suggest that thethickness of the tube wall and the interval of the nodes are notuniform. The tube wall is thin particularly in portions 203 c, 203 e.

Another carbon micro-body 207 is a carbon tube 209 grown from an Feparticle 208 in a curved manner and containing an Fe particle 213 at atip end. There are pluralities of nodes 210 in the tube 209, regionssurrounded by opposing nodes and the tube wall constituting cavities211. The Fe particle 220 near the Fe particle 213 appear to be separatedfrom each other by the tube wall 220, etc. It is not clear whether theFe particle 213 in the carbon micro-body 207 was attached to the tubefrom the beginning of growth or taken during the course of growth.Though the Fe particle 208 and the tube wall 212 look overlapping thecarbon micro-body 201, they are not connected with each other. The otherFe particle 214 exists alone. In the figure, straight or curved dashedlines 215, 216 define the profiles of carbon particles. Curved dashedlines 217, 218 define the profiles of collodion layers for fixing thecarbon micro-bodies, etc. onto a sample holder of an electronmicroscope. The reference numeral 219 denotes a scale in FIG. 12. Threelines for indicating the scale are as long as 100 nm.

FIG. 13 is an electron photomicrograph showing the structure of theparticle of the present invention, which shows the junction of the Feparticle 202 and the tube 203 in FIG. 11 in an enlarged manner. FIG. 14is a schematic view for explaining the structure shown in the photographof FIG. 13. For the simplification of explanation, FIG. 14 indicatesonly main portions of the crystal structure in FIG. 13 by solid linesand dotted lines. Accordingly, blank portions around regions of dottedlines, if any, are only omitted portions.

In FIG. 14, an Fe particle 202 has surfaces along particular crystalplanes in most regions, which are referred to as main Fe phases 220, 221below. A phase 225 with differently orientated planes, a satin-surfacephase 226, etc. were observed on the left side of the main Fe phase 220.Though the main Fe phase 220, etc. are depicted by parallel dottedlines, intervals of dots and lines are not necessarily identical withthose shown in FIG. 13.

Referring to FIG. 14, the tube 203 extends from a hypothetical boundary227 of the main Fe phase 221, and the inside of the tube wall ispartitioned by nodes 236, 239. A region surrounded by the node 236, theFe particle 202 and the tube wall, and a region surrounded by the nodes236, 239 and the tube wall constituted cavities. The tube wall wasmainly constituted by a graphite phase.

Example 3

73 g of α-Fe₂O₃ powder having an average particle size of 0.03 μm, 2.7 gof Ge powder having an average particle size of 20 μm, and 24.3 g ofcarbon black powder having an average particle size of 0.02 μm weremixed for 16 hours in a ball-milling mixer. In the above formulation,Fe/Ge was 95/5 by mass. A proper amount of the resultant mixed powdercharged into an alumina boat was heat-treated at 1000° C. for 2 hours ina nitrogen gas. After cooled to room temperature, a heat-treated powdersample was recovered.

The X-ray diffraction pattern of the above powder sample is shown inFIG. 15. Analysis by an analysis software “Jade, Ver. 5” available fromRigaku Corporation revealed that a face-centered cubic γ-Fe (111) and abody-centered cubic α-Fe (110) were identified in the diffractionpattern shown in FIG. 15. In the graph of FIG. 15, the axis of abscissasindicates a diffraction angle 2θ(°), and the axis of ordinates indicatesa diffraction intensity I [arbitrary unit (a.u.)]. The diffraction peakintensity ratio [I (111)/I (110)] is shown in Table 3. The averageparticle size of α-Fe particles was 92 nm when determined from thehalf-width. The magnetic properties of the above powder sample weremeasured by VSM. The results are shown in Table 3. The peak intensityratio was much smaller than in Comparative Example 3, suggesting thatthe sample had high saturation magnetization.

Example 4

A powder sample was produced in the same manner as in Example 3 exceptfor replacing Ge with Al. The X-ray diffraction pattern of the abovepowder sample is shown in FIG. 15. Analysis by an analysis software“Jade, Ver. 5” available from Rigaku Corporation identified aface-centered cubic γ-Fe (111) and a body-centered cubic α-Fe (110) inthe diffraction pattern shown in FIG. 15. The average particle size ofα-Fe particles determined from the diffraction peak intensity ratio andthe half-width is shown in Table 3. The magnetic properties of the abovepowder sample measured by VSM are shown in Table 3. The peak intensityratio was smaller than in Comparative Example 3, suggesting that thesample had high saturation magnetization.

Example 5

73 g of α-Fe₂O₃ powder having an average particle size of 0.03 μm, 3.8 gof vanadium carbide (VC) powder having an average particle size of 20μm, and 23.2 g of carbon black powder having an average particle size of0.02 μm were mixed, and the heat-treated powder sample was recovered inthe same manner as in Example 3. In the above formulation, Fe/V was 95/5by mass. The X-ray diffraction pattern of the above powder sample isshown in FIG. 15. The diffraction peak intensity ratio and the averageparticle size of α-Fe particles are shown in Table 3. The magneticproperties of the above powder sample measured by VSM are shown in Table3. The peak intensity ratio was smaller than in Comparative Example 3,suggesting that the sample had high saturation magnetization.

Comparative Example 3

75 g of α-Fe₂O₃ powder having an average particle size of 0.03 μm, and25 g of carbon black powder having an average particle size of 0.02 μmwere mixed without adding an element X, and a heat-treated powder samplewas recovered in the same manner as in Example 3. The X-ray diffractionpattern of the above powder sample is shown in FIG. 15. The intensityratio [I (111)/I (110)], the average particle size and the magneticproperties measured in the same manner as in Example 1 are shown inTable 3. The (111) peak intensity ratio was larger than in Examples 3 to5, suggesting that the sample had low saturation magnetization.

TABLE 3 Peak Saturation Particle Intensity Ratio MagnetizationCoercivity No. Size (nm) I(111)/I(110) (Am²/kg) (kA/m) Example 3 92 0  164 0.9 Example 4 74 0.17 132 1.9 Example 5 86 0.12 152 1.7 Comparative66 0.43 118 2.4 Example 3

Examples 6 to 9

α-Fe₂O₃ powder having an average particle size of 0.03 μm and CO₃O₄powder having an average particle size of 0.6 μm at a ratio shown inTable 4, and carbon black powder having an average particle size of 0.02μm in such an amount that it was 30% by mass were dry-mixed for 16 hoursin a ball-milling mixer. The resultant mixed powder charged into analumina boat was heat-treated at 1000° C. for 2 hours in a nitrogen gasatmosphere having a purity of 99.9% or more with an oxygen content inthe atmosphere controlled to 10 ppm or less. The average particle sizeof each powder was determined by measuring the diameters of 60 fineparticles arbitrarily selected in a TEM photograph, and averaging them.

The above powder sample was subjected to an X-ray diffractionmeasurement. Using RINT2500 available from Rigaku Corporation, themeasurement was carried out with θ/2θ scanning in a 2θ range of 40° to50° to determine the intensity of a (111) peak of a face-centered cubic(fcc) crystal and a (110) peak of a body-centered cubic (bcc) crystal.The X-ray diffraction pattern measured is shown in FIG. 16, in which theaxis of abscissa indicates a diffraction angle 2θ(°), and the axis ofordinates indicates a relative intensity of the diffraction pattern.Because overlapping diffraction patterns are not discernible, thepatterns are depicted with the scale of intensity displaced. Thediffraction pattern was analyzed by “Jade, Ver. 5,” an analysis softwareavailable from Rigaku Corporation. The diffraction peak intensity ratio[I (111)/I (110)] and the average particle size determined from thehalf-width of the (110) peak are shown in Table 5. The magneticproperties of the above powder sample measured by VSM-5 (available fromToei Industry Co., Ltd.) at a magnetic field in a range of ±2 T areshown in Table 5.

Comparative Example 4

A powder sample was produced in the same manner as in Example 6 to 9except for using 70% by mass of α-Fe₂O₃ powder having an averageparticle size of 0.03 μm, and 30% by mass of carbon black powder havingan average particle size of 0.02 μm. The X-ray diffraction pattern ofthe sample is shown in FIG. 16, and the properties of the sample areshown in Table 5.

Comparative Example 5

A powder sample was produced by the same materials (see Table 4) andproduction method as in Examples 6 to 9, except for using 70% by mass ofCO₃O₄ powder having an average particle size of 0.6 μm, and 30% by massof carbon black powder having an average particle size of 0.02 μm. TheX-ray diffraction pattern of the sample is shown in FIG. 16, and theproperties of the sample are shown in Table 5. The average particle sizeof nano-sized particles constituting the sample was determined from the(111) peak.

TABLE 4 Formulation (% by mass) No. Fe₂O₃ Co₃O₄ C Example 6 55 15 30Example 7 50 20 30 Example 8 45 25 30 Example 9 40 30 30 Comparative 70 0 30 Example 4 Comparative  0 70 30 Example 5

TABLE 5 After Heat Treatment Average Saturation Particle Magnet- Co/FeI(111)/ size ization Coercivity No. (by mass) I(110) (nm) (Am²/kg)(kA/m) Example 6 0.30 0.17 38 134 9.8 Example 7 0.44 0.12 31 139 11Example 8 0.61 0.03 27 150 12 Example 9 0.82 0.09 23 141 12 Comparative0   0.31 49 82.4 6.1 Example 4 Comparative ∞ — 98 111 3.6 Example 5

It is clear from Table 5 that the ratio of I (111)/I (110) in thesamples of Examples 6 to 9 is as small as less than 0.2, much smallerthan in Comparative Examples 4 and 5, suggesting that the addition of Cosuppresses the precipitation of a γ phase. The samples of Examples 6 to9 had as high saturation magnetization as more than 120 Am²/kg,suggesting that the addition of Co increased the volume ratio of an aphase having ferromagnetism, resulting in improved saturationmagnetization. Nano-sized particles constituting the powder samples ofExamples 6 to 9 had smaller average particle sizes than those ofComparative Examples 4 and 5, indicating that fine particles having abody-centered cubic crystal structure (α phase) were obtained.

Example 10

α-Fe₂O₃ powder having an average particle size of 0.6 μm and boronpowder having an average particle size of 30 μm were mixed in equalamounts, and heat-treated at 1100° C. for 2 hours in a nitrogen gasstream. Unnecessary non-magnetic components were removed from theproduct to obtain fine iron particles having an average particle size of2 μm each coated with boron nitride. 5 g of this fine particles weredispersed in 100 ml of ethanol, to which tetraethoxysilane was added.While stirring this dispersion, a mixed solution of 22 g of pure waterand 4 g of ammonia water was added. Thereafter, the resultant dispersionwas stirred by a ball mill, while properly adjusting the concentrationof the tetraethoxysilane and the stirring time by a ball mill. It wasthen dried at 100° C. or higher in the air, and further heat-treated at400° C. in a nitrogen atmosphere.

The resultant fine iron particles was observed by TEM and analyzed byEDX, confirming that each fine iron particle had a multilayer coatingcomprising a boron nitride layer in contact with the particle and then asilicon oxide layer outside the boron nitride layer. The boron nitridelayer was as thick as 4 nm. The thickness of the silicon oxide layer waschanged from 5 nm to 80 nm, by changing the ball-milling stirring timefrom 10 minutes to 3 hours. Also, when the amount of tetraethoxysilaneadded was changed from 0.5 g to 2 g with a ball-milling stirring timefixed to 3 hours, the thickness of the resultant multilayer coatingchanged from 5 to 80 nm. Lattice fringes assigned to a hexagonal crystalstructure were observed in the boron nitride layer. On the other hand,no lattice fringes were observed in the silicon oxide layer, suggestingthat the silicon oxide layer had an amorphous structure. The measurementof infrared absorption spectra revealed absorption peaks of siliconoxide in a range of a wave number of 1250 to 1020 cm⁻¹, confirming theformation of the silicon oxide layer.

To examine the effect of the silicon oxide layer to increase electricresistance, fine particles having a 60-nm-thick silicon oxide layeramong those having a multilayer inorganic coating obtained by the abovemethod were pressed at 20 MPa or more to form a flat plate. A mold wascoated with a silver paste for an electrode at both ends to measureelectric resistivity between the two electrode terminals. The relationbetween the thickness (nm) and resistivity (Ω·m) of the silicon oxidelayer measured above is shown in FIG. 17.

To examine the corrosion resistance of the above fine iron particleshaving a composite coating of boron nitride and silicon oxide, theirmagnetic properties were compared before and after the corrosionresistance test, which was conducted at a temperature of 120° C. and ahumidity of 100% for 12 hours by a pressure cooker test machine. Themagnetic properties were measured by VSM. Saturation magnetizationobtained by the VSM measurement before and after the corrosionresistance test, and a demagnetization ratio obtained therefrom areshown in Table 6. The saturation magnetization of the resultant finecomposite iron particles before and after the corrosion resistance testwas 64% and 61%, respectively, of that of iron.

TABLE 6 Saturation Magnetization (A · m²/kg) Demagnetization No.Before⁽¹⁾ After⁽²⁾ Ratio (%) Example 10 140 134 4.3 Example 11 148 1405.4 Example 12 152 143 5.9 Example 13 155 142 8.4 Example 15 160 152 6.9Comparative 206 165 20.0 Example 6 Note: ⁽¹⁾Before the corrosionresistance test. ⁽²⁾After the corrosion resistance test.

Example 11

Iron oxide powder having an average particle size of 30 nm and carbonpowder having an average particle size of 20 μm were mixed in equalamounts, and heat-treated at 1000° C. for 2 hours in a nitrogen gasatmosphere, to obtain fine, carbon-coated iron particles having anaverage particle size of 1 μm. The particles were then coated withsilicon oxide in the same manner as in Example 10.

The resultant fine particles were observed by TEM and analyzed by EDX toconfirm that each fine iron particle had a multilayer coating comprisingan inner carbon layer in contact with the iron particle and an outersilicon oxide layer. The carbon layer was as thick as 10 nm on average,and the thickness of the silicon oxide layer was ranging from 8 nm to 90nm. Lattice fringes assigned to a hexagonal crystal structure wereobserved in the carbon layer, confirming that the carbon layer was basedon a graphite phase. On the other hand, lattice fringes were notobserved in the silicon oxide layer, indicating that the silicon oxidelayer had an amorphous structure. The saturation magnetization of thefine composite iron particles was 68% of that of iron. The electricresistivity of the fine magnetic particles was measured in the samemanner as in Example 10. The relation between the thickness and electricresistivity of the silicon oxide layer measured is shown in FIG. 17.Corrosion resistance was also evaluated in the same manner as in Example10. The results are shown in Table 6.

One fine iron particle having a particularly small diameter particlesize among those obtained in Example 11 was observed by TEM as shown inFIG. 18. FIG. 19 is a schematic view depicting an important portion ofthe photograph of FIG. 18. The fine particle had a multilayer coatingcomprising an inner carbon layer 2 in contact with an iron core 1, andan outer silicon oxide layer 3. This carbon layer 2 was as thick asabout 5 nm, and the silicon oxide layer 2 had a thickness ranging from10 nm to 40 nm. Lattice fringes assigned to a hexagonal crystalstructure were observed in the carbon layer, confirming that the carbonlayer was based on a graphite phase. On the other hand, lattice fringeswere not observed in the silicon oxide layer, indicating that it had anamorphous structure. Surrounding smaller-diameter particles were asilicon byproduct formed separately from the fine iron particles.

FIG. 20 is a TEM photograph showing part of the particle in FIG. 18 inan enlarged manner, and FIG. 21 is a schematic view showing an importantportion of the photograph of FIG. 20. It is clear from FIG. 20 that theFe core 1 was coated with a uniform-thickness carbon layer 2.

Example 12

Fine, composite, magnetic particles each comprising a magneticiron-cobalt alloy core and a multilayer coating comprising a carbonlayer and 10-nm-thick silicon oxide layer were produced in the samemanner as in Example 11, except that iron oxide particles and cobaltoxide particles having the same average particle size, and carbon powderin an equal amount were mixed in the production of carbon-coated, finemagnetic particles. The corrosion resistance of the fine, composite,magnetic particles was evaluated in the same manner as in Example 10.The results are shown in Table 6. The saturation magnetization of theresultant fine composite particles before and after the corrosionresistance test was 70% and 66%, respectively, of that of iron.

Example 13

Fine magnetic particles were produced in the same manner as in Example11 except for using aluminum powder having an average particle size of 2μm in place of carbon powder. Electron microscopic observation and EDXanalysis confirmed that each resultant fine magnetic particle had a3-nm-thick coating layer of aluminum oxide. The fine magnetic particleswere then coated with a 5-nm-thick silicon oxide layer in the samemanner as in Examples. The saturation magnetization of the resultantfine composite iron particles was 71% of that of iron. The electricresistivity and corrosion resistance of the fine magnetic particlesmeasured in the same manner as in Example 10 are shown in FIG. 17 andTable 6.

Example 14

Fine magnetic particles were produced in the same manner as in Example11 except for using titanium powder having an average particle size of 2μm in place of carbon powder. Electron microscopic observation and EDXanalysis confirmed that the resultant fine magnetic particles had a5-nm-thick coating layer composed of a titanium compound. The resultantfine magnetic composite particles were then coated with a 50-nm-thicksilicon oxide layer in the same manner as in Example 11. The saturationmagnetization of the resultant fine composite iron particles was 65% ofthat of iron.

Example 15

Fine iron particles coated with a boron nitride layer and an outermost10-nm-thick silicon oxide layer were produced in the same manner as inExample 10. The saturation magnetization of the resultant fine compositeiron particles was 73% of that of iron. The fine composite ironparticles were dispersed in substantially neutral water, and mixed withcolloidal gold having an average size of 4 nm having a surface potentialadjusted such that its electric double layer potential (ζ potential) onthe surface was positive. Because the surface potential of the fine ironparticles was negative in neutral water, each fine composite ironparticle was coated with colloidal gold particles by sufficientstirring. The fine composite iron particles were filtered out, dried,and then heat-treated at 700° C. in a nitrogen gas atmosphere to producemultilayer-coated fine composite iron particles having a uniform coatinglayer of fine gold particles. The corrosion resistance was evaluated inthe same manner as in Example 10. The results are shown in Table 6. Themultilayer-coated iron particles were suspended in a solution of afluorescein-labeled rabbit protein (immunoglobulin). After washing, thefluorescent intensity by fluorescein was measured to calculate theamount (g) of immunoglobulin coupled to the multilayer-coated ironparticles. The relation between the concentration (g/mol) ofimmunoglobulin in the protein solution used and the amount ofimmunoglobulin coupled to the multilayer-coated iron particles is shownin FIG. 22.

Example 16

A sample was produced in the same manner as in Example 11, except that0.03 g of KCl was added as an electrolyte in the coating treatment ofparticles with silicon oxide. FIG. 23 is a TEM photograph of theresultant fine composite iron particles. FIG. 24 is a schematic viewshowing an important portion of the photograph of FIG. 23. The resultantmultilayer-coated fine composite iron particles had an inner carbonlayer in contact with the Fe core 1, and an outer silicon oxide layer 3.The TEM observation of the fine particles revealed that the siliconoxide layer was as thick as 360 nm. The saturation magnetization of thefine composite iron particles was 66% of that of iron.

Example 17

Fine iron particles each having a composite coating layer comprising aninner carbon layer and an outermost 100-nm-thick silicon oxide layerwere produced in the same manner as in Example 11. The attachment ofamino groups to the surfaces of the resultant fine particles wasconfirmed by a fluorescent-labeling method using a fluorescent substance(Rohodamine-x NHS) specifically coupling to amino groups. The finecomposite iron particles were dispersed and stirred in a 0.5-% aqueoussolution of 3-(2-aminoethylaminopropyl)trimethoxysilane for 3 hours sothat amino groups were carried by the surfaces of the fine compositeiron particles. The fine particles were then added to a solution ofRohodamine-x NHS in N,N-dimethylformamide and stirred. FIG. 25 is aphotograph of the fine particle taken by a fluorescent invertedmicroscope, and FIG. 26 is a schematic view for explaining thephotograph of FIG. 25. FIG. 26 shows fluorescence-emitting particles.This indicates that —NH₂ groups (Rohodamine-x NHS) were carried by thesurfaces of the fine composite metal particles.

Example 18

Fine composite iron particles having a multilayer coating comprising aninner carbon layer and an outermost 100-nm-thick silicon oxide layerwere produced in the same manner as in Example 11. To evaluate aDNA-extracting performance, a DNA extraction kit, “MagExtractor-Genome-®” available from Toyobo Co., Ltd., was used to carryout DNA extraction by the following procedures. First, 25 mg ofmultilayer-coated fine composite particles were dispersed in 100 μL of aTris-EDTA solution (pH 8.0), and 1 μg of DNA (630 bp) was added to theresultant fine particles dispersion. DNA was coupled to themultilayer-coated fine composite particles in a dissolving/absorbingliquid attached to the DNA extraction kit, and washed with a cleansingliquid attached to the extraction kit and a 70-% aqueous ethanolsolution. It was then stirred in sterilized water to obtain aDNA-extracted aqueous solution. In these steps, a magnetic standavailable from Promega was used to separate out the multilayer-coatedfine composite particles. The amount of extracted DNA was measured by anelectrophoresis method. The results are shown by lanes 1 and 2 in FIG.27. FIG. 28 is a schematic view for explaining the photograph of FIG.27. FIG. 28 shows a band 6 corresponding to extracted nucleic acid. Thisresult indicates that the multilayer-coated fine composite particles ofpresent invention having an outermost silicon oxide layer can extractDNA. 0.70 μg of DNA was extracted among 1 μg of DNA introduced,indicating that magnetic beads comprising the multilayer-coated finecomposite particles of the present invention have an excellent functionof extracting nucleic acid.

Example 19

Fine iron particles coated with carbon and silicon oxide and havingamino groups carried on the outermost layer were produced in the samemanner as in Example 17. DNA extraction and electrophoresis were carriedout using these fine particles in the same manner as in Example 18. Theresults are shown by lanes 3 and 4 in FIG. 27. The electrophoresisexperimental results indicate that DNA was extracted. 0.77 μg of DNA wasextracted among 1 μg of DNA introduced, 10% higher than in Example 18,indicating that the DNA-extracting performance is improved by having thefine composite particles having a carbon layer and an outermost siliconoxide layer carry amino groups.

Comparative Example 6

A silicon oxide coating layer was formed on the fine iron particlesusing carbonyl iron particles having an average particle size of 3 μm toevaluate electric resistivity and corrosion resistance in the samemanner as in Example 10. The results are shown in FIG. 17 and Table 6.Though the silicon oxide-coated, fine carbonyl iron particles obtainedin Comparative Example 6 exhibited high saturation magnetization, theyhad a demagnetization ratio of 20% in the corrosion resistance test,extremely higher than those of the multilayer-coated, magnetic, finecomposite metal particles of Examples 10 to 13 and 15. This indicatesthat the multilayer coating of the fine composite metal particles withcarbon and/or boron nitride and silicon oxide extremely improves theircorrosion resistance. The fine particles obtained in Comparative Example6 had lower resistivity than those of the multilayer-coated, magnetic,fine composite metal particles of Examples 10 to 13 and 15. This appearsto be due to the fact that the multilayer coating of carbon and/or boronnitride and silicon oxide on the fine composite metal particles provideshigh resistivity.

Comparative Example 7

A sample was produced in the same manner as in Example 11 except forusing 5 g of tetraethoxysilane, 22 g of ammonia water and 4 g of water.The silicon oxide layer had a thickness of 600 nm, and a saturationmagnetization of 100 Am²/kg, 46% of that of iron. This indicates thatwhen the silicon oxide layer is as thick as 600 nm, the saturationmagnetization decreases. This sample had large amounts of excessivesilica spheres that did not form the coating layer.

Comparative Example 8

Magnetite (Fe₃O₄) particles having an average particle size of 30 nmused as fine magnetic particles were coated with gold in the same manneras in Example 15, to measure the amount of coupled immunoglobulin of afluorescein-labeled rabbit. The results are shown in FIG. 4.

Reference Example 1

A sample was produced to measure electric resistance in the same manneras in Example 11, except for using 6.4 mol/kg (6.7 g) oftetraethoxysilane, 25.7 mol/kg (4.5 g) of ammonia water, and 79.3 mol/kg(7.1 g) of water per a unit amount of metal particles. The results areshown in FIG. 17. Though the silicon oxide layer had a thickness of 70nm and high resistivity, large amounts of silicon oxide particles wereformed immediately after the production of the sample.

It is clear from FIG. 17 that the fine particles of the presentinvention having a multilayer coating comprising a first layer of boronnitride, carbon or aluminum oxide, etc. in contact with the magneticmetal core and a second layer of silicon oxide exposed outside havehigher resistance than conventional ones. It is also clear that in theformation of the silicon oxide layer, the thickness and thus electricresistance of the oxide layer can be controlled by properly adjustingthe amount of tetraethoxysilane added. Further, as is clear from Table6, the fine particles of the present invention having multilayercoatings of carbon, boron nitride or aluminum oxide, and silicon oxidehave sufficient stability in oxidation resistance, etc.

In the present invention, the outermost surface layer may be formed by amaterial such as gold, etc. suitable for attaching to biosubstances. Asshown in FIG. 22, because the particle cores are composed of magneticmetals having high magnetic flux densities, the fine particles of thepresent invention are highly suitable for the separation andpurification of biosubstances.

As described above, the fine composite metal particles of the presentinvention are insulating and bioactive, as well as having highsaturation magnetization. For instance, the fine composite metalparticles of the present invention each comprising a magnetic metal corehaving an average particle size of 10 μm or less, and a multilayercoating comprising 2 or more layers of different inorganic materialshave high insulation with reduced deterioration of saturationmagnetization. The fine composite metal particles of the presentinvention can be produced at high productivity.

The fine composite metal particles of the present invention can be usedfor magnetic recording media such as magnetic tapes, magnetic recordingdisks, etc., magnetic shields, electronic devices such as parts forabsorbing unnecessary electromagnetic waves, filters for absorbingparticular bandwidths, inductors, yokes, etc., magnetic beads forextracting and separating DNA, protein components, cells, etc.

1. A method for producing fine composite metal particles comprising the steps of mixing oxide powder of a magnetic metal, at least one of boron-containing powder and carbon powder, and powder containing at least one element selected from the group consisting of Al, As, Be, Cr, Ga, Ge, Mo, P, Sb, Si, Sn, Ti, V, W and Zn to provide a mixed powder; heat-treating said mixed powder in a non-oxidizing atmosphere to produce fine metal particles comprising at least one of said elements and coated with a layer composed of carbon and/or boron nitride.
 2. The method for producing fine composite metal particles according to claim 1, which comprises further coating the resultant coated fine metal particles with a layer of an inorganic material.
 3. The method for producing fine composite metal particles according to claim 2, wherein the layer of said inorganic material is substantially composed of silicon oxide formed by hydrolyzing silicon alkoxide.
 4. The method for producing fine composite metal particles according to claim 3, wherein said layer of silicon oxide is formed from silicon alkoxide, water, catalyst and an electrolyte in an alcohol solvent.
 5. The method for producing fine composite metal particles according to claim 4, wherein after forming said silicon oxide layer, said fine composite metal particles are further coated with an amino-group-containing silane coupling agent to introduce an —HN₂ group to the surface of the silicon oxide layer.
 6. The method for producing fine composite metal particles according to claim 1, wherein said non-oxidizing atmosphere is composed of a nitrogen gas or a mixture of nitrogen gas and an inert gas. 